The invention is directed to an improved gas liquid contacting tray comprising a bubble area, a weir and a downcomer opening. Such trays can be used in gas liquid contacting devices. A typical use of these trays is as distillation column internals.
When trying to increase the liquid and/or gas flows in such distillation columns a maximum load will be observed. Higher loads will result in that the column fails to function as a liquid-gas contactor or separator due to a phenomena known as flooding. Flooding is described as excessive accumulation of liquid inside the column. The well known flooding mechanisms are downcomer back-up, jet flooding and downcomer choking. These mechanisms are described in Distillation Design, Henry Z. Kister, McGraw-Hill Inc, 1992, pages 267-291. According to this publication downcomer back-up is due to a build-up of liquid inside the downcomer causing the liquid to back-up on the tray leading to liquid accumulation on that tray. The liquid height in the downcomer is determined by the tray pressure drop, liquid height on the tray and frictional losses in the downcomer and downcomer slot area. Jet flooding or entrainment flooding is caused by an excessive gas velocity leading to the entrainment of liquid, either by droplets or froth, to the tray above. The liquid will accumulate, leading to flooding. Downcomer choking is caused by an excessive aerated liquid velocity in the downcomer. At a certain velocity the friction losses in the downcomer and downcomer entrance become excessive, and the frothy mixture cannot be transported to the tray below, causing liquid accumulation on the tray. The term froth is to be understood as any gas-liquid mixture present on the tray not depending on any flow regime.
U.S. Pat. No. 2,247,485 describes a tray which aims to overcome the above problem of downcomer inlet choking for a bubble cap tray by providing a vent which fluidly connects a space just below the downcomer opening and near the weir and the space above the tray. The vent as shown in the figure of said publication consists of a horizontal tube placed parallel to the weir and having openings to the space just below the downcomer opening and near the weir. The ends of the horizontal tube extend upwards and have an opening in the space above the tray. In the specification it is stressed that the vent tube is installed below the tray in order to avoid any interference with the flow of the fluid across the bubble area and the weir. U.S. Pat. No. 2,247,485 describes a tray having bubble cap openings in the bubble area of the tray. These openings result in that the predominant hydrodynamic regime above the tray, when in use, can be described as bubble regime as defined on pages 322-336 of Kister.
A disadvantage of the method as disclosed in U.S. Pat. No. 2,247,485, published in 1941, is that the vent tube is not evidently self-draining. Especially when using a gas-liquid contacting column having bubble area openings, which are typically operated in the mixed froth and/or spray regime (see also Kister pages 322-336), it is expected that more liquid droplets will be present in the downcomer and consequently enter the vent tube. Thus one skilled in the art will not readily use this vent tube because it is expected that liquid droplets will enter the vent tube and consequently disrupt the working of the vent tube. Another disadvantage is that if such a vent tube is to be used in very elongated downcomers the tube length of one vent tube will become very long. Because of the resulting pressure drop one skilled in the art will not expect an optimal working.
The problem to be solved by the present process is to provide a tray which overcomes the problem of downcomer inlet choking and which tray does not have the disadvantages accompanied with the design of U.S. Pat. No. 2,247,485.
This object is achieved with the following gas-liquid contact tray comprising a bubble area and a downcomer having a downcomer opening, wherein a self-draining vent tube is present in the downcomer and fluidly connects the upper portion of the downcomer with a space above the tray.
Applicants have found that the tray according to the invention shows a capacity improvement when compared to a tray not having a vent tube. As is clear from the description of the improved tray a vent tube is placed in a region where it could interfere with the flow of the liquid across the bubble area and the weir. It is thus surprising that a tray is obtained, having the improved capacity, in view of the teaching of U.S. Pat. No. 2,247,485 which teaching dissuades placing the vent tube in that region. A further advantage of the present invention is that the vent tube can also be used as a support, thereby simplifying the design of the tray. Another advantage is that existing trays may be simply modified by adding a vent tube to obtain a tray according to the invention. By retrofitting existing trays in this fashion a significant capacity increase of existing distillation columns can be simply achieved.